1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus using the same, more particularly to a magnetic recording medium having a recording density of 50 Gbit or more per square inch and a magnetic storage apparatus incorporating the same thereinto.
2. Prior Art
In recent years, an amount of information which computers deal with has been increased steadily, and magnetic disc apparatuses as an external storage device have been required to demonstrate larger capacity storage and a higher-speed operation. However, as a recording density of the magnetic disc apparatus becomes higher, influences of so-called thermal fluctuation thereon are remarkable. It is considered that it is difficult for conventional longitudinal recording systems to achieve an areal recording density exceeding 40 Gbit per square inch.
On the other hand, perpendicular recording systems have a characteristic that a demagnetizing field at a bit transition is low unlike the longitudinal recording systems and magnetization is kept more stably as the recording density becomes higher. Accordingly, the perpendicular recording systems are considered to be one of effective means exceeding the thermal fluctuation limit of the conventional longitudinal recording systems.
As media used in the perpendicular recording systems, there are a double-layer perpendicular recording medium having a soft magnetic underlayer and a single-layer perpendicular recording medium having no soft magnetic underlayer. For the reasons that the double-layer perpendicular recording medium can use a single magnetic pole type head capable of acquiring strong perpendicular recording magnetic field and a sharp magnetic field gradient, and that the double-layer perpendicular recording medium shows an advantage that a higher resolution can be obtained compared to the single-layer perpendicular medium, a combination of the double-layer perpendicular recording medium and the single pole type head is considered to be effective from the viewpoint of practical use.
In the case of the double-layer perpendicular recording medium, though the high resolution can be obtained, noises originated in the soft magnetic underlayer are a problem in addition to noises originated in a recording layer, which are observed in the single-layer perpendicular recording medium. These noises are roughly classified into a spike noise caused from a magnetic domain wall of the soft magnetic underlayer and so-called a transition noise which occurs by fluctuation of a magnetization transition in the recording layer owing to a magnetization state of the soft magnetic underlayer.
With regard to a method for reducing the spike noise, as disclosed in, for example, Japanese Patent Laid-Open No. 7(1995)-129946 and 11(1999)-191217, there has been a method in which a hard magnetic pinning layer between a soft magnetic underlayer and a substrate is provided to control a magnetic domain structure of the soft magnetic underlayer and thus the spike noise is reduced. However, since this conventional method does not reduce both of the spike noise and the transition noise simultaneously, the conventional perpendicular magnetic recording medium does not sufficiently reduce the medium noises from the soft magnetic underlayer.
When the adoption of the perpendicular recording systems by using the combination of the double-layer perpendicular recording medium and the single pole type head is intended so as to achieve the recording density exceeding the thermal fluctuation limit of the longitudinal recording systems, it is necessary to significantly reduce the medium noises from the soft magnetic underlayer in addition to the medium noises from the recording layer.